What is the negative of cutting the barrel of a Bruno .22 rifle? At what legth is the powder burnt out - before the bullet exit or after exit ? I’ve been told that its better to cut the barrel but then, why was the barrel then made a certain length and not shorter if it does not matter ?

Secondly smoothbore guns with barrel length of less than 20" are listed as restricted per Arms Rules. While this would not apply to rifles, in India it is best to err on the side of caution to avoid unnecessary harassment at the hands of the authorities.

AFAIK for a .22 LR complete combustion happens within the first 17 or so inches, after that any additional barrel length would actually reduce the velocity of the projectile due to friction. However, additional barrel length would also contribute towards a longer sighting plane (if using open sights) and reduction in noise, but little else.

If I were you, I wouldn't modify the Brno, as it will detract from resale value (if you were to ever sell it at a later date), without offering any great improvements in return.

"Political tags - such as royalist, communist, democrat, populist, fascist, liberal, conservative, and so forth - are never basic criteria. The human race divides politically into those who want people to be controlled and those who have no such desire." -- Robert Heinlein

The rifling of the barrel is very important aspect in modern firearms. The point where bullet begin to rotate and to the point where it exits the barrel should be of specific length and with specific rifling angle. You do not want to ruin for weapon for an experiment and weapons are not cheap in India. But if you won't experiment you will not learn the result. So please post the results if you execute this experiment.

"Political tags - such as royalist, communist, democrat, populist, fascist, liberal, conservative, and so forth - are never basic criteria. The human race divides politically into those who want people to be controlled and those who have no such desire." -- Robert Heinlein